For the tarte flambée, first combine the flour, water, oil and baking powder and knead into a smooth dough. Roll out on a baking paper.
Squeeze the lemon and mix the juice with the crème fraîche. Season with salt and pepper and brush the dough with the cream.
Cut the sweet potato into thin slices with a kitchen utensil and cut the onion into half moons. Cover the dough with it and spread the cheese and the chives evenly on it.
Bake at 180 °C for 20 minutes. Serve with salad.